Cleaner production of the lightweight insulating composites: Microstructure, pore network and thermal conductivity
Graphical abstract
Introduction
The conventional Ordinary Portland Cement (OPC) is the most common binder generally used to design low-cost lightweight insulating materials in the areas of building and construction [1], [2], [3]. However, apart from its high CO2 released during the production process, the detailed investigations show that these materials possess poor aptitude when applying as fire-resistant materials [4]. In fact, One of the challenges in the design of mass cement-based materials is to avoid the generation of cracks, which is caused by the heterogeneous distribution of temperature and stress [1]. Thermal gradients in cement-based materials can induce internal stress which leads to cracking on a microscopic or macroscopic scale [1], [4]. This explain the inefficiency of the OPC cement based insulating materials primarily due to their chemical composition and behavior in the context of variation of the temperature. The knowledge of the thermal conductivity and other thermal transport properties of construction materials involved in the process of heat transfer are essential in predicting the temperature profile and heat flow through the material [5], [6].
Many authors have demonstrated the positive impact of the amorphous or disordered aluminosilicates as fly ash, silica fume, perlite, metakaolin, etc. on the transport of the lightweight concretes [2], [3].
The porous and amorphous structure of inorganic polymer cements (IPC), often indicated as geopolymers, implies that flow in a thermal gradient will take a very tortuous route consisting of a multiple of neighboring interconnected polysialate particles [4], [7]. This is enhanced by the nanoporous nature of IPC and their thermal stability range that reach 800–900 °C [8]. The difference in the thermal conductivity between the OPC (1.5 W/(m K)) [7] and the IPC (0.6 W/(m K)) [4], [6] is linked to the chemical bulk composition, the densification, pore network and microstructure of the both matrices.
Kamseu et al. [4], [6] demonstrated how the reaction that implies the corrosion of alumina can be used to increase the insulating behavior of the IPC. It was possible to decrease the thermal conductivity of the dense IPC matrix down to 0.15 W/(m K) [4], [6]. The authors also noted the possible and significant impact of the pores coalescence into the heat transfer with the consequence on the efficiency for thermal insulation: decrease in mechanical strength and increase in permeability and thermal diffusivity.
The challenge regarding the mechanical properties, the pores coalescence and the thermal diffusivity were joined together to address the increasing demand for more environmentally friendly and sustainable materials and the desire to propose a useful recycling possibility to the natural wood fibers (saw dusts) from the wood industries in Cameroon and tropical areas motivated the authors of this work. The current work marks a significant advancement into the research of suitable eo-sustainable materials with thermos-regulator properties. These properties include heat and humid climate management between the 〈in-door and out-door〉 flux of energy through building walls in tropical regions, thereby improving human thermal conditions in buildings. According to the principle of the Kyoto Pyramid, the most cost effective method of reducing energy usage is to provide better thermally insulated buildings.
A node aim of this work is to promote building energy conservation through the construction of green buildings. Wood fibers reinforced with geopolymer binders are evidenced here as products of the clean production linked to the increase of the efficiency of resource utilization rate, wastes recycling, reduce and avoid the generation of pollutants, protect and improve environment, among others. Cleaner production simply means the continuous application of measures for design, improvement, and reduction of energy consumption, use of by-products and sustainable raw materials. The specific objective is to combine structural and thermal conductivity behavior to propose functional materials capable to isolate efficiently while withstands the mechanical and environmental solicitations (stresses).
Section snippets
Cleaner insulating matrices
Cleaner production is a preventive, company-specific environmental protection initiative. It is intended to minimize waste and emissions and maximize product output [9]. The concept was developed during the preparation of the Rio Summit as a UNEP program (United Nations Environmental Programme). The program was meant to reduce the environmental impact of industry. It built on ideas used by The UNEP's program idea was described “…to assist developing nations in leapfrogging from pollution to
Materials and preparation of the lightweight composites
Scraps wood particles (sawdust) used in this study were obtained from wood factory located in Yaoundé area, Cameroon. It is a by-product of cutting, grinding, drilling, sanding wood; it is composed of fine particles of wood with 10 μm ≤ ϕ ≤ 2 mm. The sawdust collected were dried for several months to insure the complete disappearance of humidity. The materials were then ground and sieved at 800 μm. The obtained powder presented a density of 650 kg/m3 and thermal conductivity of 0.10 ± 0.02 W/(m K) from
Physico-mechanical properties and porosity
Bind the sawdust with the geopolymer paste as described earlier willing to improve the lightweight structure and the thermal insulation of the composites leading to the intrinsic properties of the geopolymer, with thermal conductivity the half of that of OPC [1]. Fig. 2 details the variation of the bi-axial four-point flexural strength and the bulk density. It is observed that the presence of sawdust literally decrease the bulk density of the IPC composites. From 1.53 g cm−3, which is already
The effective thermal conductivity
The lightweight composite achieved under this work is a porous matrix with the microstructure homogeneous, as indicated, in the length scale of millimeters. The pores present into the matrix are homogeneous and their size did not change with the variation of the content of sawdust. At least up to the level in which the geopolymer paste completely embeds the sawdust particles. The microstructure and the pores size (ϕ < 10 μm) allow to consider only the conduction as principal heat transfer
Conclusion
Sawdust a by-product from the wood industries, abundantly available in the tropical areas of the world, was assess for the production of low cost, ecological and sustainable lightweight composite for thermal insulation. The class of sawdust used was that with the lower density (650 kg/m3), with thermal conductivity of 0.10 ± 0.02 W/(m K). This class of sawdust, inappropriate for the production of good pellets due to their lower energy, was successfully bind with metakaolin based geopolymer paste.
Acknowledgements
The authors of this article which to acknowledge the support from the Academy of Sciences for the Third World (TWAS): Grant No.: 11-024 RG/CHE/AF/AC-G; UNESCO FR:3240262695. Authors also wish to thank Dr Mirko Braga INGESSIL, Verona-Italy for the Sodium silicate.
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